Basically, battery separators serve as electronic insulators and ionic conductors, i.e. they prevent the direct electronic contact of electrodes of opposite polarity while enabling the ionic current between them. To meet these two functions, separators are usually porous insulators with pores as small as possible to prevent electronic short circuits by dendrites or plate particles and with a porosity as high as possible to minimize the internal battery resistance. In lead-acid batteries, the separator also determines the proper plate spacing and thereby defines the amount of electrolyte which participates in the cell reaction. The separator has to be stable over the life time of the battery, i.e. to withstand the highly aggressive electrolyte and oxidative environment.
Beyond these basically passive functions, separators in lead-acid batteries can also actively affect the battery performance in many ways. In valve regulated lead-acid (VRLA) batteries they additionally determine properties like oxygen transfer, electrolyte distribution and plate expansion. Due to their outstanding influence on the performance of VRLA batteries the separator is even referred to as the “third electrode” or “fourth active material” (Nelson B., Batteries International, April 2000, 51-60).
VRLA stands for valve-regulated lead-acid batteries which are also called sealed or recombinant batteries. In VRLA batteries oxygen, which is generated during charging at the positive electrode, is reduced at the negative electrode. Thus the battery can be charged and even be overcharged without water consumption and is therefore theoretically maintenance-free. The formation of hydrogen at the negative electrode is suppressed, for instance by using larger negative than positive plates in order to generate oxygen at the positive plate before the negative plate is fully charged.
For VRLA batteries two technologies are predominant, i.e. batteries with an absorptive glassmat (AGM) and gel batteries. In batteries with AGM, the absorptive glassmat immobilizes the electrolyte and simultaneously functions as a separator. In gel batteries, the acid is immobilized by means of fumed silica and an additional separator is required to fix the plate distance and to prevent electronic shorts. Compared to AGM batteries, the manufacturing cost of gel batteries is considered to be higher and their specific power is lower due to a higher internal resistance.
In AGM batteries the electrolyte is completely absorbed by the glass mat. AGM separators have a very high porosity in excess of 90%. This high porosity together with a good wettability is reflected in a very high acid absorption and low electrical resistance. In the battery, the acid saturation of AGM separators is usually in a range of 85 to 95%. This increases the effective electrical resistance versus fully saturated separators but creates open channels of relatively large pores that enable a very efficient oxygen transfer from the positive to the negative plate. The average pore size of AGM separators is usually within the range of 3 to 15 μm with an anisotropic distribution, i.e. pore sizes of about 0.5 to 5 μm in the x-y-plane of the separator which is the plane parallel to the electrode plates and pore sizes of about 10 to 25 μm in the z-direction perpendicular to the electrodes.
Due to the relatively large pores and the good wettability, the wicking rate (speed of acid pick-up) of AGM is fairly high which facilitates the filling process of batteries.
A severe disadvantage of AGM separators is their mechanical weakness which is due to the fact that pure glass separators do not contain binders of any type. The tensile strength of this material depends only on the fiber contacts and some entanglement. At the molecular level these contacts are believed to be of the hydrogen bonding type established between adjacent fibers. Since finer fibers have greater chances to establish these contacts, it follows that the strength of the material is greatly influenced by their presence.
On the other hand coarser glass fibers also play a role in the ability of the AGM separators to serve its many functions. For instance, they improve the wicking rate by creating larger pores.
This mechanical weakness of the AGM separators is even more of a problem in the light of the ongoing development of modern high-performance batteries which are characterized by steadily increasing energy densities and a reduced overall size. Accordingly the distance between the electrodes and therefore the thickness of the separators becomes thinner, further reducing their tensile strength. For an efficient and cost-effective battery production process there is a strong need for thin separators with sufficient tensile strength as to be applicable for high speed processing applications.
In an approach to benefit from both the advantages of fine and coarse glass fibers, multi-layered AGM separators have been proposed. It could be shown that two layers with fine and coarse fibers showed a better tensile strength as if these fibers would have been dispersed in one sheet (Ferreira A. L.; The Multilayered Approach for AGM Separators; 6th ELBC, Prague, Czech Republic, September 1998).
U.S. Pat. No. 5,962,161 discloses separators made from a mat of meltblown ultrafine polymer fibers which may be reinforced with one or more thin layers of spunbond fabric.
U.S. Pat. No. 4,908,282 discloses fibrous sheet separators comprising a mixture of glass fibers and polyethylene fibers.
It also has been suggested to include microporous sheets as part of the separator system in order to control mechanical properties of the separator. An example for this is the use of a layer of microporous polymer material for improving the compression behavior of an AGM separator by arranging the polymer layer between two layers of AGM (Weighall M. J.; ALABC Project R/S-001, October 2000). Favorable compression/recovery properties have been shown to be important since the application of high plate group pressures via a separator having a low compressibility can eliminate premature capacity loss and extend the life of the battery. An example of such a microporous separator is a microporous PVC separator having a mean pore size of 5 μm and a thickness of 0.57 mm, sandwiched between two layers of AGM with a thickness of 0.52 mm at 10 kPa (Weighall M. J., see above; Lambert U., A study of the effects of compressive forces applied onto the plate stack on cyclability of AGM VRLA batteries, 5th ALABC Members and Contractors' Conference Proceedings, Nice, France, Mar. 28-31, 2000).
This separator configuration might provide for improved mechanical properties when compared to AGM separators. However, the presence of the microporous layer hampers the ionic current between the electrodes, thereby increasing the internal electrical battery resistance. This is disadvantageous for applications which do usually not involve deep discharge cycles, such as starter batteries or stand-by emergency power batteries. Moreover, due to the outer AGM layers these separators are difficult to form into pockets. Moreover, due to the thickness of the polymer membrane these separators are not applicable for spiral wound cells.